Prepolymer compositions which thermally cure into solidified adhesives and coatings should desirably be one part compositions. Multiple part adhesive and coating compositions must be carefully measured, thoroughly mixed, and promptly used, requirements which are inconvenient and may cause waste of materials.
Many adhesive and coating compositions must be dissolved or dispersed in an organic solvent before they can be applied to a workpiece. These solvents are environmentally hazardous and their use should be avoided whenever possible, see "Adhesion and Bonding", Enc. of Polymer Science and Technology, 1, 486, Interscience, 1964.
Processing applications such as printed circuit manufacture often employ "stageable" adhesives, that is, adhesive compositions which can be partially cured to a tack-free coating, fastened to an adherend, and cured using heat, pressure, or both (see. U.S. Pat. No. 4,118,377). The tack-free state is sometimes referred to as the "B-Stage".
Several compositions have been described which embody some but not all of the features of one part formulation, freedom from solvents, and stageability. For example, blocked polyisocyanates which can be used as one-part thermosetting adhesive or coating compositions have been described in Levine and Fech Jr., J. Org. Chem., 37, 1500 (1972), Levine and Fech Jr., J. Paint Tech., 45 (578) 56 (1973), U.S. Pat. No. 3,808,160, and British Patent Specification Nos. 970,459 and 1,085,454. However, these compositions begin to cross-link upon being heated to their deblocking temperature, and therefore cannot be readily B-staged.
U.S. Pat. No. 3,435,003 discloses a method for cross-linking a saturated condensation polymer backbone bearing furan groups by reacting it with a bis-maleimide. The starting polymer backbone is a solid at room temperature, and is mixed with the bis-maleimide by dissolving the polymer in a solvent with heating or by heating the polymer to its melting point. Upon cooling, the composition forms a solid and is said to cross-link to its cured state within a few days at room temperature. The composition of that patent would therefore be difficult to formulate as a one part, stageable composition.